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Service composition is considered to be an effective way for resource sharing and node collaboration in Mobile Ad hoe NETworks (MANETs) environment,In order to improve the reliability and shorten the response time of composite services,this paper first analyzes the node environments and network environments that affect the component services1 availability,and then proposes an Environment-aware Quantitative Evaluation Model for Service Availability (EQEM-SA) In addition,based on EQEM-SA.a service field concept is proposed as well as the availability attenuation equation,where the availability value is considered as the field strength.Then the service selection approach based on service field model is presented.The effective ness of the proposed approach is illustrated and compared with rehted references,and the results of experimental evaluations indicate that our approach significantly reduces the failure rate and shortens the service delay.